AP Galio actually is insane. Don't try to defend it.
It's not just that guy who makes PBE montages. This happened in a game I was playing.
baron steal + triple kill https://youtu.be/hVZMXi1A7pk
Notice that he's actually invincible during his taunt, which he uses at two points during that clip. That's what happens when you have additive damage reduction. The EHP gain is exponential.
Galio is capable of reaching literally 100% damage reduction but even with like 80% or so it's already too much. Since when do AP nukers get an Alistar ult as a normal skill?
It would be safer to change the skill to armor/MR so that it gives linear EHP scaling.
Other than that, the damage is quite ridiculous. Here's the damage chart from that game: https://puu.sh/y6ByV/18119ba1f5.png